Structure for independently supporting each strand of a medially-looped filament in a vacuum tube



Sept. 17, 1968 J. w. KENDALL. JR.. ETAL 3, ,3

STRUCTURE FOR INDEPENDENTLY SUPPORTING EACH STRAND OF 7 A MEDIALLY-LOOPED FILAMENT IN A VACUUM TUBE Filed March 10. 1966 2 Sheets-Sheet 1 ZZZ/WWW mmmxx INVENTORS JACKSON W KENDALL,JR.

Jomv L2 MACGOWAN FIG! A M ATTORNEY J. w. KENDALL. JR.. ETAL. 3, ENDENTLY SUPPORTING EACH STRAND OF Sept. 17, 1968 STRUCTURE FOR INDEP A MEDIALLY-LOOPED FILAMENT IN A VACUUM TUBE Filed March 10. 1966 2 Sheets-Sheet 3 FIGS 72a 72b FIG3 FIG.4

FIG.2

INVENTORS JACKSON WKENDALL. JR JOHN D. MACGOWN ATTORNEY United States Patent 3,402,312 STRUCTURE FOR INDEPENDENTLY SUPPORTING EACH STRAND OF A MEDIALLY-LOOPED FILA- MENT IN A VACUUM TUBE Jackson W. Kendall, Jr., Salt Lake City, Utah, and John D. MacGowan, Los Altos, Calif., assignors to Varian Associates, Palo Alto, Calif., a corporation of California Filed Mar. 10, 1966, Ser. No. 533,176 6 Claims. (Cl. 313-278) ABSTRACT OF THE DISCLOSURE Structure in a vacuum tube for independently supporting each strand of one or more medially-looped filaments under tension in order to keep the strands straight during thermal cycling. The support structure comprises two resilient members each of which support one strand. The medial loop of the filament bridges the two resilient members in an untensioned condition thereby permitting each member to support one strand independently.

This invention relates to a structure for supporting a vacuum tube filament and more particularly, for supporting a filament of the type forming a directly heated cathode in such tube.

Supports for vacuum tube electrodes, including filaments, desirably support the electrodes in fixed spaced relation to one .another throughout all operating conditions to which the tube is subjected. Close spacing between the grid and the filament or cathode is particularly important in constructing tubes having high transconductance. Typical electrode supports provide fixed lower supports to which the lower ends of all filament and grid wires are fastened, an upper support to which the upper ends of .all such wires are fastened, and a structure for resiliently biasing the upper support away from the lower support to tension uniformly all wires. In directly heated cathode structures, however, the cathode or filament Wires are necessarily subjected to high temperatures and the individual filament wires tend to distort as a result of thermal expansion .and contraction. Because of inevitable variations in the properties and size of the various filament wires in a given tube and because of inevitable variations in tension applied to the various filament wires during fabrication of the filament structure, at least some of the filament wires in a multi-filament tube bow during the operating life of the tube. Not only does such bowing adversely affect electrical characteristics in directly heated cathode tubes, but such bowing can cause a short circuit between the grid and cathode of the tube and consequent destruction of the tube. The present invention overcomes the aforementioned disadvantages by providing a filament support structure that aflords independent tensioning force to each filament wire so that throughout all operating conditions of the tube, the individual filament strands will be properly tensioned and therefore will be maintained in a straight unbowed position. A vacuum tube employing the filament support structure of this invention substantially avoids inter-electrode short circuits and permits closer spacing between the grid and the directly heated cathode, thereby increasing the transconduetance of the tube.

The structure exemplifying the present invention and described in more detail hereinafter includes one or more hairpin-shaped filaments, i.e., filaments formed of a unitary length of wire having a medial loop so as to define two individual filament strands, which are electrically and mechanically terminated at the free ends thereof in a conventional manner. The improved supporting structure is engaged on the filament adjacent the looped end 3,40Z:,3 l2 Patented Sept. 17, 1968 of the hairpin, each individual strand of the filament hairpin being independently supported on a resilient member with the filament loop bridging the two resilient arms in an untensioned condition. The untensioned loop permits the resilient arms to tension each of the Strands independently and affords a small degree of movement to each of the arms without impairing the electrical continuity between the two strands afforded by the loop.

An object of the present invention is to provide a structure for independently supporting each individual strand of a filament structure having one or more hairpin-shaped filaments to the end that differing forces in each filament strand arising from thermal expansion are independently compensated. This object is achieved by a structure having an individual arm or tab for each filament strand which arms or tabs are mounted to afford a slight degree of resilient, bendable movement. Two strands are joined to adjacent tabs and the loop between the strands spans the side of the structure opposite the strands in an untensioned condition so that either or both of the tabs can move to maintain the desired tension or the filament strands.

Another object is to provide a vacuum tube electrode structure in which a grid structure can be mounted extremely close to the cathode structure. Achievement of this object permits construction of vacuum tubes with improved electrical characteristics and having lower input power requirements because of the fact that the magnitude of the grid input signal is inversely proportional to the distance between the grid and cathode electrodes. The object is achieved by forming the cathode of plural filamentary wires, e.g., those formed of thoriated tungsten or the like, and by installing the wires in fixed parallel relationship in such a way that the wire cannot bow or distort when subjected to stresses arising from thermal expansion and contraction.

A further object is to provide a filament mounting structure having properties for inherently compensating for minor variations in filament tension that inevitably arise during fabrication of the tube. Because of the resilient tabs that support one end of each filament strand, the individual filament strands are maintained in tension throughout the operating ranges of the tube. A consequence of the resilience or bendability of the tab is that somewhat less care during fabrication of the tube is necessary. Consequently, fabrication is speeded up and the costs of fabrication are reduced, while at the same time providing a tube with superior electrical characteristics.

Other objects of the invention will be more apparent on referring to the following specification and accompanying drawings in which:

FIG. 1 is a side elevation view in partial cross section of a high power vacuum tube employing the filament structure of the present invention, portions being broken away for clarity;

FIG. 2 is an elevation view at enlarged scale of the upper filament support of the tube of FIG. 1;

FIG. 3 is a top view of FIG. 2;

FIG. 4 is an enlarged view of an upper filament support showing modification of the present invention; and

FIG. 5 is a top view of the structure of FIG. 4.

Referring more particularly to the drawings, a vacuum tube employing the present invention includes a base 12 on which is mounted a lower electrode supporting structure 14 and an upper electrode supporting structure 16. The upper electrode supporting structure is mounted on the upper end of a sleeve 18 that is supported on a rod 19, extending upwardly of base 12. At the lower end of sleeve 18 a spring 20 is disposed in circumscribing relation to rod 19 to upwardly resiliently bias sleeve 18 relative base 12. The resilient biased connection formed by spring is a conventional expedient for effecting installation of upper electrode supporting structure 16 such that the electrodes are subjected to a tensioning force to retain the electrodes in a fixed spacial position. The exemplary tube also includes an envelope 22 mounted to base 12, the envelope supporting in a conventional manner an external anode 24 provided with a plurality of cooling fins 26 radiating outwardly therefrom.

Interior of anode 24 are disposed concentrically thereof a filament or cathode electrode 28, a control grid electrode 30 and a screen grid electrode 32. Conventionally, each of the electrodes is formed of a plurality of parallelly extending wires arranged in a generally cylindric configuration concentric with one another and with sleeve 18. The wires constituting grids 30 and 32 are not subjected to excessive temperatures during operation so that the mounting of these wires in a uniformly tensioned condition can be accomplished in accordance with prior art techniques with relative ease. Control grid 30 is electrically connected by conventional techniques to a terminal ring or flange 34 mounted on the base 12, and screen grill 32 is connected to a similar conductive ring or flange 36.

Filament electrode 28 is typically formed by a plurality of thermionic wires such as thoriated tungsten or the like which when heated by the passage of current therethrough will emit electrons and act as the cathode of the tube structure. Referring to FIG. 2, filament structure 28 is formed of a plurality of hairpin shaped conductors, each including a pair of strands 38 joined at the upper end by a medial loop portion 40. The lower or free ends of strands 38 are joined to respective base support members 42 and 44 which define alternating rows of spaced tabs 46 to each of which the lower end of a respective strand is joined by welding or the like. In accordance with conventional practice, members 42 and 44 are electrically connected to terminal rings or flanges 46 and 48, respectively. (See FIG. 1.)

Disposed on the upper end of sleeve 18 is a collar 50 slidably circumscribing rod 19 and having an integral, generally circular flange 52 extending laterally therefrom. Collar 50 constitutes an element of upper electrode support structure 16 and is biased upwardly of lower electrode support structure 14 through the action of spring 20 and sleeve 18. Supported on flange 52 is a plurality of spring clips 54 which are formed as integral radial extensions of a central disc 56. Each clip is curved to define an outer hook portion 58 for engaging medial loop of the filament. Overlying disc 56 is a saddle 60 formed of resilient high temperature resisting material, the saddle being formed to define a plurality of radially extending tabs 62. Shown in FIG. 3, by way of example only, is a saddle having twelve tabs 62 extending therefrom in uniformly spaced relation around the circumference of the saddle. Because each tab 62 is thin in a vertical direction and relatively wide in a horizontal direction, a point on its outer edge will have only one degree of freedom of movement in a direction axially of strand 38. Each tab is formed on its outer edge with a notch 64 in which an individual filament strand 38 is supported to space the filament strand radially and circumferentially in a uniform cylindric pattern. Saddle 60 is oriented on flange 52 so that spring clips 54 register with alternate inter-tab spaces and so that curved portion 58 of the hook extends above the saddle. Accordingly, each loop 40 is engaged on a spring clip 54 thereby retaining the strand 38 of the filament in respective notches 64.

As can be noted most clearly in FIG. 3, loop 40 is so formed that individual strands of a given filament hairpin element can move relative one another. Such characteristic is achieved by forming the portions of each loop 40 between a notch 64 and clip 54 in a generally arcuate or curved portion. Thus, as individual tabs 62 bent downwardly by different amounts in response to different tensions on individual filament strands 38, the

arcuate or curved portion of loop 40 assumes a relatively straight or relatively curved configuration in proportion to the amount of downward movement of the associated tab. Accordingly, the integrity and electrical continuity of the medial loop 40 is not interrupted even though one strand of a particular hairpin loop may be subject to more or less movement than the opposite strand. The resilience of spring clip 54 contributes to such relative movement between adjacent strands 38.

In assembling a filament structure according to the present invention the lower ends of filament strands 38 are secured to tabs 46 by welding or the like, with the upper extremity of medial loop 40 of each individual filament hairpin being approximately equispaced above lower electrode support 14. Individual strands are then engaged in respective notches 64 and the media loop is passed over a respective spring clip 54 for engagement thereby. When full tension on the filament strands is applied by release of spring 20, collar 50 is resiliently biased upwardly and saddle 60 is correspondingly biased. Each filament strand is uniformly tensioned through the cooperation of the arcuate portion of loop 40 above saddle 60, the resilient character of each tab 62, and the resilience afforded by spring clip 54. When the filament is initially fired, the extreme high temperatures thereof frequently effect a weld-like connection between individual strands 38 and the walls of tabs 62 that define notches 64. However, the resilience residing in each tab and the arcuate portion of the medial loop 40 permit a certain degree of resilient movement between individual strands 38 as the individual strands are subjected to varying forces due to thermal expansion and contraction. Accordingly, each individual filament strand is maintained in a straight precisely aligned position throughout all operating ranges of the tube as a consequence of which control grid 30 can be spaced somewhat closer to the filaments without danger of a filament-grid short circuit.

Referring to FIGS. 4 and 5, another form of the upper filament support structure of the present invention includes a saddle 70 mounted atop flange 52 in generally the same manner as described hereinabove with respect to FIG. 2. Saddle 70 includes a plurality of radially extending tabs 72, each tab having at the outer extremity thereof a bifurcated portion formed by furcations 72a and 72b. Saddle 72 has the same physical properties as saddle 62 described hereinabove, namely: resilience and capability of withstanding high temperatures. Each furcation 72a, 72b has formed in the lateral edge thereof a notch 74 for receiving a filament strand 38 therein. Medial loop 40 spans the bifurcated portion of tab 72 in an untensioned, upstanding condition so as to afford relative movement between furcations 72a and 72b. The width of tabs 72 and the spacing between adjacent tabs 72 are preferably established so that strands 38, when engaged in notches 74, are uniformly spaced around the electrode structure.

In fabricating a filament structure employing the embodiment of FIGS. 4 and 5 the lower ends of strands 38 are suitably connected to terminals 46 of lower electrode support 14, and the upper portions of the strands adjacent medial loop 40 are engaged in grooves 74 and joined thereto by any suitable process, such as heliarc welding. The force of spring 20 is then permitted to become effective to upwardly bias saddle 70 through sleeve 18 and flange 52. As can be seen in exaggerated form in FIG. 4, the individual furcations at the outer extremities of tabs 72 are subject to different degrees of flexure so as to accommodate differing amounts of tension on filament strands 38 arising from slight variations during fabrication of the filament structure and arising from variations due to thermal expansion and contraction during operation of the tube. Because medial loop 40 is in an untensioned and upstanding position, independent movement of furcation 72a with respect to furcation 72b is permitted; the portion of tab 72 intermediate the main body of saddle 70 and the bifurcated portion of the tab permits both furcations to experience an additional amount of movement. In one structure designed in accordance with this invention as embodied in FIGS. 4 and 5 a differential in length of adjacent strands 38 equalling about .015 to .020 inch was compensated for by the filament support structure. Although the embodiment of FIGS. 4 and 5 requires somewhat more care in fabricating the filament structure as compared with the embodiment of FIGS. 2 and 3, it is somewhat less complex in construction and affords the desired uniform tension in each individual filament strand. Therefore, all of the strands will be retained in a straight, unbowed position throughout all operating ranges to which the tube is subjected.

Thus it will be seen that the present invention provides an improved and novel structure for mounting in a vacuum tube electrode structure a directly heated cathode formed by one or more hairpin-shaped filaments. By use of the present invention fabrication of the cathode struc ture is simplified and expedited. Moreover, the individual filament wires are maintained in a straight and unbowed condition to the end that the space between the cathode electrode and the control grid electrode can be very closely controlled without liability to inter-electrode shorts. The close control of such cathode-grid spacing affords improved and more uniform electrical characteristics in tubes employing the filament supporting structure of the invention.

Although several embodiments of the invention have been shown and described it will be apparent that other adaptations and modifications can be made without departing from the true spirit and scope of the invention.

What is claimed is:

1. In a vacuum tube, a filament structure comprising at least one filament wire centrally looped to define two strands, means rigidly attaching the ends of said wire in the tube, said attaching means affording electrical connections to the respective filament ends, a pair of tabs mounted in spaced relation to said attaching means, each tab defining a notch for receiving a filament strand there in and supporting each strand in fixed spaced relation to the other strand, each tab being individually resiliently biased away from said attaching means in a direction axially of said strands, the looped portion of said filament spanning said tabs in an untensioned condition to afford relative movement of said tabs.

2. In a loop type filament support for a vacuum tube having filaments formed of first and second strands joined by a medial loop, means supporting the free ends of said first and second strands, a pair of resilient tabs each engaged with one'gf'of said first and second strands a predetermined distarice below said loop, said tabs positioned to maintain said first and second strands in a fixed predetermined relatior'i to each other, and each of said tabs being resiliently movable only in an axis parallel with the longitudinal axis of said strands.

3. A filament supportaccording to claim 2 and wherein said strands are mounted on the outside edges of respective tabs, said pair of tabs and said loop extends upwardly over in bridging relation to said tabs in free standing relation.

4. A filament support according to claim 3 wherein said free end supporting means comprises a plurality of strand terminations arranged in a circular configuration, a saddle having a plurality of resilient radial extensions having bifurcated resilient outer portions, each bifurcated portion forming a pair of associated tabs, each said tab defining a notch on the lateral edge thereof remote from the associated tab, said notches being arranged in a circular configuration substantially congruent with said strand terminations, portions of said strands adjacent said loop being fixed in respective said notches so that each said loop extends in untensioned bridging relation over said bifurcated portion.

5. A filament support according to claim 2 and wherein said strands engage the end edges of said fingers and said loop extends therefrom in substantial juxtaposition to the tabs in bridging relation thereto and means including a resilient clip engaging said loop formaintaining said loop in juxtaposition to said fingers.

6. A filament support according to claim 5 wherein said free end supporting means comprises a plurality of strandterrnjnations arranged in a circular configuration,

a"saddle' having a plurality of resilient radial extensions defining said tabs, each tab defining a notch at the o'titer extremity thereof, said notches being arranged in a circular configuration substantially congruent with said" terminations, said loop maintaining means being disposed"? in alternate inter-tab spaces adjacent said saddle.

References Cited UNITED STATES PATENTS 2,385,435 9/1945 Werner et al. 313-278 2,441,349 5/1948 Eitel et al. 3l3-272 X 2,490,786 12/1949 Deutsch 3l3--278 X JOHN W. HUCKERT, Primary Examiner.

A. I. JAMES, Assistant Examiner. 

